This invention relates generally to the field of filament wound objects, and more particularly to such objects which are elongated, relatively thin in cross-section and tapering, and are used primarily to support objects a distance above the ground, such as for example utility poles used to support lines or lights. Even more particularly, the invention relates to such objects which are composed of incremental or segmental portions which are co-axially aligned, where a first mandrel is filament wound, a second mandrel is aligned with and abutted against the first and both mandrels are filament wound, then successive mandrels are added as desired, such that an object with non-uniform wall thickness is produced, and where the entire process is completed before any of the individual windings cure so that the object cures as a unitary member with no weaker secondary bonds.
Filament winding is a reinforced plastic process employing a series of continuous resin impregnated fibers applied to a rotating mandrel in a predetermined geometrical pattern under controlled tension, which then cures to form a body with a high strength-to-weight ratio, good corrosion, thermal and impact resistance, and a high strength-to-thickness ratio. The filaments may be composed of fiberglass, graphite, aramid or the like. Suitable resins include epoxies, polyesters, polyimides, silicones and phenolics. Mandrels may be made of cardboard, wood, plastic or metal, and may in some instances be removed from the filament wound object after curing or left in place.
Cylindrical objects such as fiberglass reinforced plastic pipe and tanks have been made by the filament winding process for years. Equipment for this process resembles the conventional machine shop lathe. The mold or mandrel is positioned between the headstock and tailstock and is rotated so that continuous reinforcement, saturated with plastic binding material, may be pulled onto the surface. The carriage which dispenses the reinforcement moves in a direction parallel to the longitudinal axis of the mandrel. The linear speed of the carriage is synchronized with the surface speed of the mandrel so that the reinforcement is applied at some predetermined and controlled position and orientation. The carriage traverses back and forth for whatever length of travel is required to build the length of the part. The number of circuits of carriage travel and rotations of the mandrel establish the amount of material deposited onto the mandrel and thereby the thickness of the part.
The synchronization of carriage speed to mandrel surface speed establishes the angle at which reinforcement is applied and thereby the properties of the part being constructed on the mandrel. If carriage speed is fairly slow when compared to mandrel rotation, then reinforcement is applied more near the circumferential direction. This type of filament winding is commonly referred to as hoop winding. If carriage speed is fast compared to mandrel rotation, then reinforcement is applied more near the axial direction. This type of filament winding is commonly referred to as polar, longitudinal or axial winding. Winding at intermediate speed ratios between these two extremes will apply the reinforcement in helical patterns on the mandrel surface and is called helical winding. Helical winding closer to the axial direction is called low angle while helical winding closer to the circumferential direction is called high angle.
The choice and distribution of winding direction and thickness controls the tensile, flexural, compressive, elastic, rigidity, bearing and other mechanical properties of the finished product. Hoop winding is the easiest fabrication method and provides maximum circumferential strength. Polar winding provides maximum strength parallel to the cylinder axis but presents two problems not encountered in hoop winding. First, the reinforcement must be held onto the mandrel when carriage travel is reversed at the end of the carriage stroke. This may be accomplished by winding over the ends of the mandrel so that the reinforcement is mechanically held and prevented from sliding toward the mandrel center. Another method is to stop the carriage at the end of the stroke and allow the reinforcement to wind onto the mandrel in the hoop direction to effectively tie itself onto the mandrel. This carriage operation is referred to as dwell. One of more revolutions of the mandrel may be required in the dwell position to fix the reinforcement and a substantial build-up of material may occur. The second problem encountered in polar winding involves sagging of the reinforcement away from the mandrel. This is particularly troublesome with polar winding of long objects.
Helical winding provides the designer with a method to establish and control properties in the circumferential and axial directions by applying reinforcements at some angle in between the hoop and polar directions. The weakness in this process occurs at the ends of the mandrel where the direction of carriage travel is reversed. Sliding of reinforcement may occur and is usually overcome by the same means as in polar winding.
Filament wound poles which have a slight taper are known and are used in many applications where a load is to be supported a distance above the ground. A typical example is a utility pole, which can be up to 35 feet or more in length. Tensile properties are to be maximized in this type of object, meaning that polar winding would be generally preferred as it places the high strength reinforcement in the most useful direction. But because the pole is generally vertically oriented, consideration must be given to the compressive strength at the base, meaning that the wall must be of sufficient thickness at and near the base end of the pole to support the remainder of the pole itself and the load carried by the pole. Horizontal loads applied by wind and wires cause deflection and stresses that are primarily axial in direction. The stresses are tensile on one side and compressive on the other. Failure of a pole generally occurs because the compressive strength of the thin wall is exceeded, resulting in a buckling of the pole. If the pole is only polar wound, then achieving the required base thickness will result in excessive thickness in the upper part of the pole, with excessive overall weight and material waste. If the pole is polar wound only to the thickness required in the upper portion of the pole, then additional reinforcement material must be added at the base. If only helical winding is used and the wall thickness is varied from the base to the tip of the pole, then winding angle must be progressively increased from the tip to the base, with accompanying loss of axial strength as the winding angle increases. This may require the addition of some additional tensile reinforcement toward the end of the pole to meet structural demand.
Regardless of whether the pole is polar wound or helically wound, the purpose is to most efficiently use the high tensile strength of the reinforcement to satisfy the tensile strength requirement of the pole. However, thin wall cylindrical shapes loaded in this fashion frequently fail by compressive buckling long before the full tensile strength is developed or exceeded. this is particularly true of composite materials with low modulus of elasticity values. To increase resistance to compressive buckling, stiffeners may be spaced at suitable intervals as a part of the product. Known prior art techniques involve either wrapping or applying the stiffening elements externally to the wound pole, which is not aesthetically pleasing, or involve removing the mandrel and inserting bulkhead members into the interior of the pole, which are then fastened adhesively. It is very difficult to obtain a good secondary bond between the interior wall of the pole and the stiffeners in this manner.
It is an object of this invention to provide a filament wound pole, and a novel method for making such as pole, which is filament wound primarily in the polar or low angle helical direction and has a greater wall thickness at the base of the pole than at the tip, the increased thickness being due to the presence of additional layers of filament wound reinforcement. It is a further object to provide such a pole which is wound as one integral piece on multiple mandrels, coaxially aligned in incremental steps to create the total longitudinal length of the pole, where a first mandrel is wound with reinforcement, a second or upper mandrel is coaxially aligned and abutted against the first mandrel, and both mandrels are then filament wound, such that the first mandrel has two layers of reinforcement and the second mandrel has one, with successive upper mandrels and windings applied in like manner to increase the number of reinforcement layers and extend the pole as required. It is a further object to provide such a pole where the mandrels are dimensioned such that the outer diameter of the base end of each mandrel aligned to a previously wound mandrel is approximately equal to the outer diameter of the reinforcement surrounding the previously wound mandrel, such that the surface of the outer layer of added reinforcement applied to both mandrels is smooth at the junction between the added mandrel and the previously wound mandrel. It is a further object to provide such a pole where the incremental winding steps are all successively performed prior to curing of any individual winding, such that the finished pole cures as a single, unitary member with no secondary bonding between components. It is a further object to provide such a pole with integral internal circumferential stiffeners created during the fabrication process at strategic locations to provide resistance to premature compressive buckling failure.